Article describes how a DVC positioner can be used to allow users to take credit for an unexpected plant trip as a SIL interlock proof test, and how it can also detect impending issues that could signal a degrading safety shutdown valve condition.
By Stanley Amirthasamy and Christina Ng
Stanley Amirthasamy, Senior Product Manager for Fisher FIELDVUE Instruments at Emerson Automation Solutions, and Christina Ng, Certified Functional Safety Expert (CFSE) at Fluor Control Systems, published an article in the January 2020 issue of Processing highlighting the unexpected benefits of utilizing DVC positioners on safety shutdown valves. Their article is titled Valve Proof Test Credit for a Process Trip and is summarized below.
Process Trips
Unplanned process trips are never welcomed in a plant that runs continuously. The authors describe the situation as follows:
When a process trip occurs, the main goal is usually to restart the unit or equipment that has been shut down as soon as possible. Taking the opportunity to proof test an automated valve will not be a top priority or even an activity under consideration due to the tight schedule after an unplanned shutdown.
However, when a trip occurs that activates the entire safety interlock loop successfully, the event can often be considered a SIL loop proof test. How can proof test credits be claimed for a process trip? Stanley and Christina explain:
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns due to actual demand on the SIS during operation may be given credit as proof tests (fully or partial) under given conditions…the next planned proof test may be skipped.”
These conditions are:
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test can be considered performed. A sample list of activities performed during a proof test, along with those that are performed during a process trip, is shown in Figure 1. Even without an automated valve leak test, data captured by the DVC alone can potentially account for most of the proof test coverage for an automated valve.
See Figure 1 below for a list of the activities performed during a proof test and data that can be automatically captured in a DVC positioner.
No.
Activities Performed in a Proof Test
Activities Completed in a Process Trip
1
Test Procedure Signed and Dated
2
Leak Test Performed
3
Visual Inspection Performed
4
Manual Functions Tested
(e.g. Bypass, ESD, Override)
5
User-Initiated Diagnostics Performed (e.g. Deviation, BADPV, Failure Direction)
6
SIF Response Time Validated
x
7
Sensor(s) Tested
8
Logic Solver / Logic Function Tested
9
Final Element(s) Tested
10
Alarm(s) Tested
11
Failure Data Recorded
12
Faulty Devices Repaired/Replaced
Figure 1: Proof Test vs Process Trip. Data captured by a Fisher DVC6200SIS digital valve controller during a process trip meets 7 of the 12 requirements of a proof test.
Stanley and Christina explain further that:
If the process trip takes place within a predetermined maximum time window, the end user may choose to leverage the process trip as a proof test by completing steps one through five in Figure 1, which are usually not completed in a process trip.
Other Unexpected Benefits
While a proof test credit is certainly valuable, there are other benefits provided by the DVC positioner that are often overlooked. ISA lists the following most common root causes of a shutdown valve performance degradation.
Any of these issues could inhibit a shutdown valve from performing its function and result in a very hazardous condition, but Stanley and Christina tell us:
All of these conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 2).
Figure 2: Data during a process trip is captured by the Fisher DVC6200SIS digital valve controller and made available for analysis.
Diagnostic data captured during a process trip may reveal valve degradations that may not be detected during a proof test. For example, diagnostic data captured during a process trip might indicate a problem with the valve closing completely against the full pressure of the process, which can be an early indication of valve leakage.
Clearly data in the DVC can be very valuable for detecting impending issues that may not keep the valve for performing now but could create problems in the future.
Summary
The authors summarized their article as follows:
A process trip can provide the coverage required to delay a valve’s next scheduled proof test, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process trip as a proof test. Even if the end user chooses not to take proof test credits for a process trip, the valve diagnostic data provided by the DVC can help plant personnel make proactive valve maintenance decisions.